tracing: Fix synth_event_add_val() kernel-doc comment

It's named field here.

Link: https://lkml.kernel.org/r/20210516022410.64271-1-hqjagain@gmail.comSigned-off-by: NQiujun Huang <hqjagain@gmail.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_synth.c

@@ -1978,7 +1978,7 @@ EXPORT_SYMBOL_GPL(synth_event_add_next_val);
 /**
  * synth_event_add_val - Add a named field's value to an open synth trace
  * @field_name: The name of the synthetic event field value to set
- * @val: The value to set the next field to
+ * @val: The value to set the named field to
  * @trace_state: A pointer to object tracking the piecewise trace state
  *
  * Set the value of the named field in an event that's been opened by